Ring lasers (i.e., lasers where the internal light never propagates back on itself), as is known, provide a higher extraction efficiency (i.e., power output of the laser compared to power available in the gain medium), from the gain medium than typical standing wave (linear) resonators or coupled standing wave resonators (i.e., lasers where the internal light propagates back on itself).
It is also known in the art of ring lasers that two or more ring lasers or resonators may be coupled together (i.e., phase-locked, or having a constant phase difference between beams) by a technique such as that described in U.S. Pat. No. 4,841,541 to Sziklas et al., entitled "Coupling of Ring Lasers". The patent discusses two coupling embodiments to couple a plurality of ring lasers together, each ring laser having an output beam that has a constant phase difference between the other lasers in the array.
More specifically, the Sziklas patent shows, in a first embodiment, reverse-to-forward wave coupling in which a portion of the reverse wave of one laser is injected (or coupled) into the forward wave of another laser. In a second embodiment, the patent shows forward-to-reverse wave coupling in which a portion of the forward wave of one laser is injected (or coupled) into the reverse wave of another laser.
In the reverse-to-forward wave coupling embodiment, a "forward wave suppressor" mirror is used to couple the forward wave into the reverse wave of the same laser by injecting a portion of the forward wave output beam of the laser into the reverse wave. Similarly, in the forward-to-reverse wave embodiment, a "reverse wave suppressor" mirror couples the reverse wave to the forward wave of the same laser.
Also, it is known that to provide a viable way of transmitting high-energy laser beams to a distant target, it is easier to propagate several smaller beams to the target than one large beam. This is especially true when the beams travel long distances through the atmosphere causing thermal blooming which limits the amount of power that can be carried in a single beam.
Typically, each output beam from the coupled lasers are fed to a focusing mechanism, e.g., a phased array telescope, to expand the laser beam, to allow focusing at long distances, and to point and focus the laser beam on the target. However, it is necessary for all the beams to be in phase at the target in order to deliver the maximum intensity (or irradiance).
If one of the focusing devices moves due to vibration, thermal, or other effects, the distance from the focusing device to the target changes. Furthermore, if the distance to the target changes by half a wavelength of the light transmitted, the phase of one beam relative to the others changes such that the interference pattern seen at the target changes from having one bright fringe in the middle of the pattern (when all beams are in phase) to having fringes not in the middle of the pattern. Consequently, the power is divided between these fringes, and thereby reduces the irradiance on the target.
To prevent this out-of-phase effect, if caused by focusing devices, requires adjusting the focusing devices which typically comprise large water-cooled mirrors designed for high-powered beams. Because of the large size of the mirrors, obtaining fine control is difficult.
Alternatively, if changes occur in the path length of the cavity of one of the ring lasers, e.g., due to vibration or other effects, the output beam from the laser with the cavity change may become out of phase with the other output beams. To prevent this out-of-phase condition, the large cavity mirrors must be adjusted and, due to the large size, fine control is again difficult to achieve.
Therefore, it is desirable to provide a coupled ring laser system that can alter the phase of the output beam of one laser without having to control large focusing means, without having to control large cavity mirrors, and without affecting the phase of the other lasers in the system.